In a starter with a planetary gear speed reduction mechanism of the prior art, as shown in FIG. 38, an output shaft 220 is arranged at one end side with a flanged protrusion 361 having a larger diameter than the external diameter of the output shaft 220 and in its outer circumference with a groove 220a, in which is fitted a washer 10. The flanged protrusion 361 is formed with a plurality of holes, in which are press-fitted pins 332. These pins 332 support a planetary gear 320 rotatably through a metal bearing 333. The planetary gear 320 meshes with not only an internal gear 360a formed in the inner circumference of a center bracket 360 but also a sun gear 310 formed on a drive shaft 510.
Further, the output shaft 220 is in its axially backward movement regulated as a washer 10 fitted in the groove 220a of the output shaft 220 comes into abutment with the front end face of a small diameter cylindrical portion 365 arranged in the front end position of the center bracket 360 and as the rear end face 360b of a large diameter cylindrical portion 366 of the center bracket 360 comes into abutment against a motor partition 800.
In the starter having the aforementioned planetary gear speed reduction mechanism, however, in case the output shaft receives an excessive axially backward load from a ring gear of an internal combustion engine through a pinion gear, the center bracket having an external diameter larger than that of the output shaft arranged between the washer fitted in the groove of the output shaft and the motor partition is pushed from the both axial sides by the washer and the motor partition. Since in the center bracket the small diameter cylindrical portion which abuts the washer on the output shaft and the large diameter cylindrical portion which abuts the motor partition wall are disposed away from each other in a radial direction, the wall portion connecting the two cylindrical portions deforms receiving loads axially oppositely from the radially outermost end and the radially innermost end. As a result, the internal gear formed on the inner circumference of the center bracket is deflected to invite a defect that it cannot retain its satisfactory meshing engagement with the planetary gear.
Therefore, the present invention has been conceived to solve the above-specified problem and has an object to provide a starter with a planetary gear speed reduction mechanism, which can regulate the axially backward movement of an output shaft reliably.